TUNIS - Tunisian President Moncef Marzouki on Thursday testified in the murder probe of Chokri Belaid, his office said in a statement, after the opposition leader's family said he had received death threats.

"President Moncef Marzouki received this morning the judge of the Tunis first instance tribunal who heard his testimony as a witness in the assassination of Chokri Belaid," said a brief statement.

The family of Belaid, who was gunned down outside his Tunis home on February 6, say the president was aware the vocal critic of the ruling Islamist party Ennahda had received threats before his murder.

But presidential spokesman Adnene Mancer denied these claims when they were first made following the assassination.

"My brother had told me, and I quote him: 'The presidency has contacted me to tell me that I am threatened with death. And the president offered me protection but I refused because it would be tantamount to controlling me'," he said.

"As far as I am concerned, the president knows who ordered the murder, those who decided to kill Chokri," he added.

Tunisia says radical Salafist Muslims murdered Belaid.

Interior Minister Ali Larayedh, who is also prime-minister designate, said Tuesday the killer of the leftist opposition leader had been identified and four suspects arrested.

Those held were "Tunisians and belong to a radical religious strand... which we refer to as the Salafist movement," said Larayedh.

But some family members have dismissed any involvement of Salafists in the assassination, insisting Ennahda was responsible -- a claim the Islamist party staunchly denies.

Belaid's murder sparked deadly street protests and strikes and exposed the widening fissures between the ruling Islamists and liberals.

It also threw Tunisia into its worst political crisis since the revolution two years ago that ousted strongman Zine El Abidine Ben Ali, prompting prime minister Hamadi Jebali to resign last week.